This is a Continuation of International Application PCT/EP01/11073 filed Sep. 25, 2001 which in turn claims priority of German application DE 100 49 842.6 filed Oct. 9, 2000, the priorities of which are hereby claimed, said International Application having been published in German, but not in English, as WO 02/32196 A1 on Apr. 18, 2002. International Application PCT/EP01/11073 is incorporated by reference herein in its entirety, as if fully set forth herein.
1. Field of the Invention
The present invention relates to a circuitry arrangement for the operation of at least two gas discharge lamps.
2. Description of the Related Art
Through the employment of so called double-lamp or multiple-lamp ballasts to a certain extent a reduction of the outlay in terms of circuitry can be achieved. The advantage in comparison with the employment of ballasts which in each case control only a single gas discharge lamp consists in that a greater part of the components of the ballast, for example the rectifier, the harmonics filter, the control circuit and the inverter, can be simultaneously employed for the operation of a plurality of lamps.
The inverter and the load circuit of a known double-lamp ballast, which is disclosed in EP 0 490 329 A1, are schematically illustrated in FIG. 4 and will be briefly explained below. The inverter is formed by means of two controllable switches S1 and S2 which are arranged in a half-bridge arrangement to the input of which a supply d.c. voltage VBUS is applied. The two switches S1 and S2 are so controlled by a control circuit 1 that they alternatingly open and close so that at the middle point of the half-bridge there is yielded a high frequency a.c. voltage Uac. This a. c. voltage is delivered to the load circuit, which initially on the input side has a series resonant circuit of an inductance La and a capacitance Cr. To the common node point between the inductance La and the capacitance Cr, the two gas discharge lamps LA1 and LA2 are connected in parallel in each case via a coupling capacitor Ck1 and Ck2.
Further, there is connected upstream of two gas discharge lamps LA1 and LA2 a balancing transformer Lbal, the windings of which are flowed through by the two lamp currents. This happens in opposite senses so that upon deviations of the current amplitudes a magnetization arises which induces a voltage in the windings, which in turn works in a balancing manner. By means of the balancing transformer Lbal component tolerances and lamp tolerances, and different temperature conditions, which could have the consequence that the two lamps LA1 and LA2 burn with different brightnesses, can be compensated to a certain degree.
The balancing effect of the transformer Lbal is however restricted and does not ensure a complete equalization of the lamp currents. For example at low currents, which occur with small dimming levels, the lamps are practically parallel connected, since the voltage drop at the balancing transformer can amount only to a fraction of the arc drop voltage of the lamps. This is manifest particularly at lower temperatures, where the arc drop voltage at small lamp currents reaches a maximum.
This case is illustrated in FIG. 5. Thereby, the two lamps are to be operated at a brightness which corresponds to a certain desired current ISOLL. However, due to tolerances, the two lamps are not identical but manifest characteristic lines Uarc1 and Uarc2 which are slightly displaced with respect to one another, as they are illustrated in FIG. 5. Thus, for example, with a predetermined current, the second lamp requires in principle a somewhat greater arc drop voltage Uarc2 than the first lamp. In order then to be able to operate both lamps with the desired current, ISOLL, two different arc drop voltages USOLL1 and USOLL2 would be necessary. Since, however, the ballast with the inverter makes available only one voltage value USOLL1, which in the illustrated example is determined by the lamp having the lower arc drop voltage, that is by the first lamp having the characteristic line Uarc1, this voltage USOLL1 is also applied to the second lamp. As a consequence thereof the second lamp does not take up the desired current value ISOLL but possibly forms a second working point with a different current value Iarc2 and therewith naturally also has a different brightness. There exists, however, also the danger that the second lamp having the higher arc drop voltage possibly may be able to find no fixed working point and as a consequence extinguishes.
In order therefore, in the case of lower brightness values, to avoid the extinguishing of one of the two lamps LA1 or LA2, there is effected with the ballast illustrated in FIG. 4 the regulation of the inverter always in accordance with that lamp LA1 or LA2 which has the lower lamp current at the time. For this purpose, the ballast has two detection circuits 21 and 22 which in each case detect the current flowing through a lamp LA1 or LA2, in that they determined the voltage dropped across a measurement resistance RSENS1 or RSENS2. The actual values VIST1 and VIST2 generated by the two detection circuits 21 and 22 are then delivered to a comparator circuit 3 which selects the corresponding lower value and passes this as the final actual value VIST to the control circuit 1 for the control of the inverter.
Thus, there is needed for each lamp its own detection circuit, in order to be able reliably to ensure that neither of the two lamps extinguishes. The outlay in terms of circuitry is, however, again increased through this. Further, it is to be taken into consideration that depending upon the switching capacitances of the lamps or the wiring, a capacitive current always also flows through the lamps. A satisfactory control is, however, only then ensured if the actual effective component of the lamp current is determined. For this purpose complex and expensive circuits are necessary. Finally with the multiple lamp systems, with which more than two lamps are connected to a single inverter, there is needed a complex selection circuit for selecting the lowest actual value in each case.
It is thus the object of the present invention to indicate a simplified circuitry arrangement for the operation of at least two gas discharge lamps, with which the extinguishing of one of the lamps can be reliably avoided.
This object is achieved by means of a circuitry arrangement in accordance with the present invention. In accordance with the invention, n (n is a whole number and greater than 1) gas discharge lamps are operated with a single inverter, which is supplied with a d.c. voltage and generates an a.c. voltage which is alterable in its frequency, which is delivered to a load circuit arrangement at the output of the inverter. Thereby, the load circuit includes a series resonant circuit of an inductance and a capacitance, and the n gas discharge lamps connected to the common node point between the inductance and the capacitance. Further, the load circuit contains (nxe2x88x921) balancing transformers for the balancing of the currents of in each case two gas discharge lamps.
In order to prevent that one of the lamps extinguishes, in accordance with the invention the load circuit has for each gas discharge lamp a d.c. current supply line which in each case taps between the output side terminal of the winding of the balancing transformer and the gas discharge lamp and via which a d.c. current is delivered to each gas discharge lamp. Thus, each gas discharge lamp receives, along with the a.c. voltage delivered via the resonant circuit and the rectifier, additionally an independent current source which supplies the lamp with a d.c. current. This additional d.c. current corresponds advantageously approximately to the half of the nominal 1% current at 25xc2x0 C. to 35xc2x0 C. It has the effect that even for the case that due to the predetermined a.c. voltage no stable working point can develop, no lamp extinguishes. Beyond this, the additional d.c. current prevents the appearance of so-called running layers.
The d.c. current supply lines have preferably in each case a resistance connected in series with the lamp and are connected at their input side terminal to a common supply voltage. This supply voltage can be obtained for example with the aid of a diode connected to the output of the inverter, whereby preferably between the diode and the d.c. supply lines there is arranged a capacitor connected with ground.
Through the measures in accordance with the invention, the extinguishing of the lamps can be reliably prevented. However, due to asymmetrical wiring capacitances and lamp capacitances, great brightness differences can arise since the balancing transformer or transformers tend to equalize the relatively great currents and as a consequence in a lamp having lesser wiring capacitance an additional effective current is generated. In order to avoid this and to attain a better balancing of the lamp currents, in accordance with a further development of the invention the two windings of a balancing transformer can in each case be connected with one another by means of a series circuit of a capacitor and a resistance. This has the consequence that the balancing effect of the transformer is reduced for small lamp currents, without thereby the d.c. current sources being affected. The reduction of the balancing effect manifests itself solely on the a.c. current components of the lamp voltage, that is only on that part which at small dimming levels is substantially influenced by asymmetric wiring capacitances.
The circuitry in accordance with the invention distinguishes itself in that it can be expanded in simple manner from a double lamp system to a multiple lamp system. Further, it is no longer necessary to provide for each lamp its own detection circuit for the measurement of the lamp current. Rather, it is sufficient to employ solely a single detection circuit, which detects the sum of the effective powers of the gas discharge lamps arranged in the load circuit and generates a corresponding actual value. On the basis of a comparison between this actual value and a predetermined desired value, the inverter can then be controlled. The detection of the sum of the effective powers can, for example with a half-bridge rectifier, be effected in simple manner in that the voltage dropped across a measurement resistance arranged at the base point of the half-bridge is determined.
The d.c. supply lines proposed in accordance with the invention, with the resistances connected in series to the lamps, which are connected on the input side to a common supply voltage, can also be employed with multiple lamp lamp systems with which no balancing transformers are provided.